Google Browser Sync: This awesome plugin from Google keeps bookmarks, saved form data/passwords, history, cookies, and even your most recent set of open tabs on their servers and keeps all linked Firefox browsers synchronised!

Google Toolbar: The Google Toolbar is finally available for Firefox. My favorite features are the search-term highlight, the auto-form-filling (for address, phone, etc), and spell check of text-box entry.

Google Notebook: The Google Notebook allows you to define notebooks and save text and pictures in a notebook as you are browsing the web that you can later review.

Google Calendar Notifier: I love the Google calendar, and this puts event notifications and daily summaries so that you can see them from the Firefox status bar.

Gmail Space: Presents an in-browser WUFTP-style interface to your 2GB+ of Gmail disk space where you can store files that you may need to access from various computers.

Download Statusbar: Download progress is shown on a special download status bar at the bottom of the browser instead of the pop-up download manager.

Wizz RSS: A side-bar RSS news reader. The nice thing is that with Google Browser Sync above the items you have read/not read are synced across browsers.

FasterFox: Allows you to tweak various settings in Firefox for a faster browser experience, including pre-fetching, and some things that violate some RFCs.

VideoDownloader: Allows you to save videos from sites such as YouTube and Google Video (including AVI format there).

DownThemAll: Tool to allow you to conveniently download multiple items from a page

TabMixPlus: Allows you to customize everything about tabs. In particular I like the feature to open new tabs next to the current tab, the red X on each tab, the loading progress bar on each tab, the ability to protect tabs, and the fact that it shows which tabs you have not yet looked at. Also includes a session manager which I disable due to my preference for Google Sync.

PDF Download: Convienient little plugin that gives you an easy choice to view PDFs in the browser, outside the browser, or download it.